A great variety of interactive computer systems and computer input devices useful therewith are known in the art. Various types of light pens are described in the following U.S. Pat. Nos., which are believed to represent the current state of the art: 5,691,749; 5,600,348; 5,187,467; 5,179,368; 5,151,688; 5,138,304; 5,134,389; 4,923,401; 4,812,828; 4,802,722; 4,789,836; 4,697,175; 4,677,428; 4,675,665; 4,642,459; 4,620,107; 4,602,907; 4,565,947; 4,454,417; 4,377,810; 4,367,465; 4,277,783; 4,263,592; 4,190,831; 4,146,880; 4,129,858; 4,109,146; 4,053,765; 4,017,680; 3,997,891; 3,944,988; 3,758,717; 3,659,281; 3,651,509; 3,594,608; 3,551,896; 3,543,240; 3,506,875; 3,505,561.